Silver-containing photothermographic imaging materials (that is, photosensitive thermally developable imaging materials) that are imaged with actinic radiation and then developed using heat and without liquid processing have been known in the art for many years. Such materials are used in a recording process wherein an image is formed by imagewise exposure of the photothermographic material to specific electromagnetic radiation (for example, X-radiation, or ultraviolet, visible, or infrared radiation) and developed by the use of thermal energy. These materials, also known as “dry silver” materials, generally comprise a support having coated thereon: (a) a photocatalyst (that is, a photosensitive compound such as silver halide) that upon such exposure provides a latent image in exposed grains that are capable of acting as a catalyst for the subsequent formation of a silver image in a development step, (b) a relatively or completely non-photosensitive source of reducible silver ions, (c) a reducing composition (usually including a developer) for the reducible silver ions, and (d) a hydrophilic or hydrophobic binder. The latent image is then developed by application of thermal energy.
In photothermographic materials, exposure of the photographic silver halide to light produces small clusters containing silver atoms (Ag0)n. The imagewise distribution of these clusters, known in the art as a latent image, is generally not visible by ordinary means. Thus, the photosensitive material must be further developed to produce a visible image. This is accomplished by the reduction of silver ions that are in catalytic proximity to silver halide grains bearing the silver-containing clusters of the latent image. This produces a black-and-white image. The non-photosensitive silver source is catalytically reduced to form the visible black-and-white negative image while much of the silver halide, generally, remains as silver halide and is not reduced.
In photothermographic materials, the reducing agent for the reducible silver ions, often referred to as a “developer”, may be any compound that, in the presence of the latent image, can reduce silver ion to metallic silver and is preferably of relatively low activity until it is heated to a temperature sufficient to cause the reaction. A wide variety of classes of compounds have been disclosed in the literature that function as developers for photothermographic materials. At elevated temperatures, the reducible silver ions are reduced by the reducing agent. In photothermographic materials, upon heating, this reaction occurs preferentially in the regions surrounding the latent image. This reaction produces a negative image of metallic silver having a color that ranges from yellow to deep black depending upon the presence of toning agents and other components in the imaging layer(s).
Differences Between Photothermography and Photography
The imaging arts have long recognized that the field of photothermography is clearly distinct from that of photography. Photothermographic materials differ significantly from conventional silver halide photographic materials that require processing with aqueous processing solutions.
In photothermographic imaging materials, a visible image is created by heat as a result of the reaction of a developer incorporated within the material. Heating at 50° C. or more is essential for this dry development. In contrast, conventional photographic imaging materials require processing in aqueous processing baths at more moderate temperatures (from 30° C. to 50° C.) to provide a visible image.
In photothermographic materials, only a small amount of silver halide is used to capture light and a non-photosensitive source of reducible silver ions (for example a silver carboxylate or a silver benzotriazole) is used to generate the visible image using thermal development. Thus, the imaged photosensitive silver halide serves as a catalyst for the physical development process involving the non-photosensitive source of reducible silver ions and the incorporated reducing agent. In contrast, conventional wet-processed, black-and-white photographic materials use only one form of silver (that is, silver halide) that, upon chemical development, is itself at least partially converted into the silver image, or that upon physical development requires addition of an external silver source (or other reducible metal ions that form black images upon reduction to the corresponding metal). Thus, photothermographic materials require an amount of silver halide per unit area that is only a fraction of that used in conventional wet-processed photographic materials.
In photothermographic materials, all of the “chemistry” for imaging is incorporated within the material itself. For example, such materials include a developer (that is, a reducing agent for the reducible silver ions) while conventional photographic materials usually do not. The incorporation of the developer into photothermographic materials can lead to increased formation of various types of “fog” or other undesirable sensitometric side effects. Therefore, much effort has gone into the preparation and manufacture of photothermographic materials to minimize these problems.
Moreover, in photothermographic materials, the unexposed silver halide generally remains intact after development and the material must be stabilized against further imaging and development. In contrast, silver halide is removed from conventional photographic materials after solution development to prevent further imaging (that is in the aqueous fixing step).
Because photothermographic materials require dry thermal processing, they present distinctly different problems and require different materials in manufacture and use, compared to conventional, wet-processed silver halide photographic materials. Additives that have one effect in conventional silver halide photographic materials may behave quite differently when incorporated in photothermographic materials where the underlying chemistry is significantly more complex. The incorporation of such additives as, for example, stabilizers, antifoggants, speed enhancers, supersensitizers, and spectral and chemical sensitizers in conventional photographic materials is not predictive of whether such additives will prove beneficial or detrimental in photothermographic materials. For example, it is not uncommon for a photographic antifoggant useful in conventional photographic materials to cause various types of fog when incorporated into photothermographic materials, or for supersensitizers that are effective in photographic materials to be inactive in photothermographic materials.
These and other distinctions between photothermographic and photographic materials are described in Imaging Processes and Materials (Neblette's Eighth Edition), noted above, Unconventional Imaging Processes, E. Brinckman et al. (Eds.), The Focal Press, London and New York, 1978, pp. 74–75, in Zou et al., J. Imaging Sci. Technol. 1996, 40, pp. 94–103, and in M. R. V. Sahyun, J. Imaging Sci. Technol. 1998, 42, 23.
Problem to be Solved
Historically, photographic films containing various silver halides have been used for various radiographic purposes. Desired sensitivity to imaging X-radiation has been achieved through amplification of a relatively small number of latent image centers without too much “noise” being added to the image. However, such films require the use of undesirable aqueous processing solutions and equipment.
The term “noise” is understood in radiography to refer to the random variations in optical density throughout a radiographic image that impair the user's ability to distinguish objects within the image. Radiographic noise is considered to have a number of components identified in the art as described for example by Ter-Pogossian, The Physical Aspects of Diagnostic Radiology, Harper & Row, New York, Chapter 7, 1967.
Wet-processed radiographic films have generally been used in combination with metal plates or metal oxides that convert X-radiation to electrons, or inorganic phosphors that convert X-radiation to visible radiation. Such “converting” materials are also usually provided in a separate element in what is known as “metal screens”, “intensifying screens”, or “phosphor panels” because if phosphors or metal oxides are included within the typical silver halide emulsion, very high image noise levels result. Thus, metal or phosphor intensifying screens or panels have been commonly used in combination with radiographic films in what are known as cassettes or radiographic imaging assemblies.
Thus, attempts to incorporate phosphors in wet silver halide to improve sensitivity to X-radiation have been not been favored. K. Becker and coworkers found that incorporation of p-terphenyl into a wet silver halide emulsion gave a material with a flat energy response between 10 keV and 1000 keV but with an excessive amount of noise (K. Becker, E. Klein, and E. Zeitler, Naturwissenschaften, 1960, 47, 199, K. Becker, Roentgenstr, 1961 a, 95, 694, and K. Becker, Roentgenstr, 1961b, 95, 939).
U.S. Pat. No. 4,865,944 (Roberts et al.) describes “unitary” intensifying screen and radiographic elements in which layers of silver halide emulsion and phosphor-containing layers are coated adjacent to each other in conventional “wet” processed photographic materials.
Efforts have been made to increase photographic speed in photothermographic materials because such materials offer a number of important advantages over the use of conventional wet-processed photographic materials. Photographic speed can be increased in a number of ways including the use of various chemical sensitizing compounds; However, the use of such compounds may sometimes diminish the “natural age keeping” properties of the photothermographic materials, whereby Dmin tends to increase in unexposed areas over time.
In addition, because the level of silver halide is relatively low compared to wet-processed photographic materials, direct exposure of such materials to X-radiation would require that an undesirably high dosage be delivered to the film (through a patient) in order to produce a useful image.
One approach to reducing the amount of X-radiation exposure needed to produce an image in photothermographic materials is to place “double faced coatings” of photothermographic materials into contact with metal or phosphor intensifying screens [see for example JP Kokai 2001-109101 (Konica) and JP Kokai 2001-022027 (Konica)].
Advances in the art are provided with the X-radiation sensitive photothermographic materials described in U.S. Pat. No. 6,440,649 (Simpson et al.) and U.S. Pat. No. 6,573,033 (Simpson et al.) in which various phosphors are incorporated into imaging layers containing chemically sensitized silver halides.
However, there is a continuing need to find ways to improve the natural age keeping of solvent-based, high-speed X-radiation-sensitive photothermographic materials.